The art of heat exchange has been active for hundreds of years. One type of heat exchanger that has evolved over this time period is one that utilizes so-called tube to header joints. In this type of heat exchanger, two headers are typically located in spaced parallel relation. Each header is provided with a plurality of tube receiving apertures and the apertures in one header are aligned with corresponding apertures in the other. Tubes extend between and have their ends received in the headers. The ends are also sealed to the headers and then tanks are fitted to the headers in sealed relation to receive and confine a heat exchange fluid passing from the tank and header on one end of the assembly through the tubes to the tank and header on the other end of the assembly. In some instances, one or more baffles may be employed to provide for so called multi-passing.
Whatever the particular flow path arrangement, it is common to place fins between the respective tubes. When so-called flattened tubes are used, it is customary to utilize so-called serpentine fins while when round tubes are employed, and in some instances even with flattened tubes, plate fins may be employed as well.
In use, many of these heat exchangers have intermittent duty cycles, which is to say that a heat exchange fluid of a temperature higher or lower than that of the temperature of another heat exchange fluid is passed through the tubes from one header to the other as mentioned previously.
As a result, dimensional changes in the tubes and headers occur as a result of the heating or cooling of the tubes and the header and the resulting thermal expansion or contraction. Where the tubes are bonded to the headers, such thermal cycling induces stresses at the tube to header joints. These stresses in turn ultimately cause fatigue which is generally concentrated in the walls of the tube (since tube walls are typically thinner than headers and header flanges which may receive the ends of the tubes) until a fracture results causing leakage, and thus failure of the heat exchanger.
Such failure is highly undesirable. In the case where heat exchanger may be repaired, the system in which it is used must be necessarily shut down for a sufficient period to allow the repair to be undertaken. Where the heat exchanger cannot be repaired, the same problem is present plus there is the additional cost of providing an entirely new heat exchanger to replace that which has failed. Consequently, heat exchanger failure due to thermal cycling is highly undesirable and should be avoided.
In many incidences, the prior art, to reduce thermal cycling failure, has simply resorted to using heavier components as, for example, tubes and/or headers of greater thickness. While this approach works well, it adds to the cost of the heat exchanger because greater thicknesses mean that more material must be employed in the fabrication of the heat exchanger, thereby raising material cost. In addition, weight is increased and in various intended uses, as for example vehicular applications, weight is desirably reduced rather than increased to achieve better fuel economy.
Another solution is proposed in PCT patent publication WO 03/093751 A2 published on Nov. 13, 2003. In this patent document, an insert whose periphery is complimentary to the periphery of the interior of the heat exchanger tubes at their ends is inserted into the ends of the tubes so as to be present at the tube header joints and provide additional strength to resist fatigue imposed by thermal cycling.
While little is known about the effectiveness of this proposal, it has at least one clear drawback. That is that each insert must be assembled to a tube end in individual operation thereby increasing assembly costs.
Furthermore, it is believed by the present applicants that its implementation utilizes more material than is actually required to attain the goal of increasing the thermal cycle life of a heat exchanger.
The present invention is directed to overcoming the foregoing difficulties.